ECG measuring instruments are primarily used for measuring and monitoring a patient's cardiac function, for which purpose the summation voltage of the electrical activity of the myocardial fibers is typically measured in the form of what is termed an “ECG signal” by way of at least two electrodes. An ideal waveform of such an ECG signal is shown by way of example in FIG. 1 as voltage U over time. According to Einthoven characteristic waveforms of the ECG signal are designated by the letters P, Q, R, S and T and generally reflect the different phases of a heartbeat.
There are other applications besides the pure monitoring of a patient's cardiac function. For example, ECG signals are also used in medical imaging applications for the purpose of generating trigger signals. Information about the cardiac cycle is acquired via the ECG signal during imaging in order thereby to synchronize the imaging with the cardiac activity. In particular in the case of imaging methods that require a relatively long acquisition time, high-quality images of the heart or images of regions that are moved by the heartbeat can be produced in this way.
ECG measuring instruments are also used for in-situ recording of ECG signals during an examination of a patient by means of a magnetic resonance device. In this case, however, operation in the magnetic resonance device imposes special requirements on the ECG measuring instrument due to the strong gradient fields and radio-frequency fields used there for the imaging in order to prevent mutual interference between magnetic resonance device and ECG measuring instrument. ECG measuring instruments that are magnetic-resonance-compatible in the aforementioned sense are available on the market.
Identifying R waves in ECG signals is essential for reliable triggering. Said identification is, however, made more difficult e.g. as a result of T-wave overshoots occurring in the magnetic field.
Magnetic fields that change over time, as used in the magnetic resonance device as magnetic gradient fields for position encoding, also continue to represent a further major problem in relation to reliable ECG signal measurement. According to the law of induction, temporally fluctuating magnetic fields of said type generate interference voltages which are coupled into the ECG signal recorded by the ECG electrodes as noise. Magnetically generated interference signals of said kind become superimposed on the ECG signal generated by the heart and distort said signal.
These sources of interference are extremely undesirable. Reliable detection of the R wave of the ECG signal is necessary in order to synchronize an acquisition of a magnetic resonance image with the heartbeat. The noise signals can be erroneously interpreted as an R wave e.g. due to their often similar shape and consequently can incorrectly initiate a triggering of an acquisition of a magnetic resonance image. On the other hand it can also happen that a “real” R wave is not detected as such due to the superimposed noise signals. This frequently leads to a significant deterioration in image quality.
Prior art attempts to solve these problems consisted in subjecting signals interpreted as a possible R wave to a simple threshold value check in addition prior to a triggering. Said threshold value check generally provides a maximum value that is not to be exceeded and a minimum value that is not to be undershot. If the maximum value is exceeded, it is assumed that a parasitic induction was present due to the gradient fields. If the minimum value is undershot, it is assumed that a T wave has erroneously been interpreted as an R wave. In both cases no trigger signal is issued.
Changes to the measurement conditions for the ECG measuring instrument represent a further difficulty. For example, in certain MR examinations it is necessary for the patient being examined to hold his/her breath for a certain time. This also affects the measured ECG signal, which can make it even more difficult to detect an R wave.